BACKGROUND: Digital technologies have been widely acknowledged as a potentially useful resource for increasing mental healthcare access. The working alliance is a key influence on outcomes in conventional psychotherapy, but little is known about therapists' experiences of forming an effective working alliance in blended interventions that involve in-person psychotherapy and a digital programme. AIMS: To investigate psychological well-being practitioners' (PWPs') experiences of the working alliance in a trial of blended cognitive-behavioural therapy (b-CBT) for depression. Trial registration ISRCTN12388725. METHOD: Semi-structured qualitative interviews were conducted with 13 PWPs who delivered b-CBT in a two-arm, non-inferiority randomised controlled trial investigating the effectiveness of b-CBT compared with face-to-face CBT. Thematic analysis was used to analyse the data. RESULTS: Participants reported four facilitating factors when building and maintaining a working alliance in b-CBT: having more time to deliver treatment, access to a wider toolkit, capacity to tailor components of b-CBT and receiving appropriate training and support. Participants also identified four barriers to building and maintaining a working alliance: time and resource constraints, usability challenges, limited flexibility to tailor the digital programme to patients' needs and lack of confidence in delivering b-CBT. CONCLUSIONS: Our study is the first specifically to investigate practitioners' perceived facilitators and barriers to forming a working alliance in b-CBT for depression. Findings suggest that PWPs' experiences of the working alliance can be improved by: accounting for the time required to deliver b-CBT in service workflows to reduce time pressures; increasing opportunities to tailor the digital programme through offering transdiagnostic tools and adaptable features; and providing appropriate b-CBT training and technical support.